Impact of Pressure on Low-Molecular Weight Near-Critical Mixtures of Limited Miscibility

Near-critical mixtures of limited miscibility are significant for chemical physics, soft matter physics, and a variety of challenging applications. Their basic properties can be tuned by compressing or a systematic change of one of the components. This report addresses these issues, based on experimental studies in nitro-compound (nitrobenzene, o -nitrotoluene, and 1-nitropropane) and n -alkane (from pentane to eicosane) critical mixtures. Studies reveal new patterns for the evolution of the critical consolute temperature ( T C ) and concentration ( x C , mole fraction) within the tested homologous series: T C ( n ) ∼ n 2 and x C ( n ) ∼ n 1/2 . They also show two paths of the high-pressure impact: (i) d T C ( P )/d P > 0 and overlapping of normalized T C ( P ) dependences and (ii) the crossover d T C ( P )/d P < 0 → d T C ( P )/d P > 0 with increasing n -alkane length. The consistent parameterization of all T C ( P ) dependencies is introduced. Supplementary nonlinear dielectric effect studies indicate a possible molecular origin of the phenomenon. The coexistence curve under high pressure is in the agreement with the isomorphism postulate for critical phenomena but with a surprisingly strong distortion from the Cailletet-Mathias law of the rectilinear diameter. The new and reliable method for estimating the critical concentration and temperature is proposed. It explores the analysis of relative volumes occupied by coexisting phases.